1. Field of the Invention
The present invention relates to a water-dispersible binder composition which is suitable for the production of a stoving filler to be processed from aqueous phase, to a process for the production of this stoving filler and to its use for the production of chip resistant coatings.
2. Description of the Prior Art
The production of chip resistant paint coatings is of considerable interest, particularly in the automotive field. Previously, solvent-containing coating compositions have largely been used for this purpose. For example, DE-OS 3,337,394 describes a chip resistant paint based on polyesters and blocked isocyanates which contains typical organic solvents for paints. However, for economic reasons and for reducing environmental pollution, increasing attempts are being made to avoid organic solvents in paints to the extent possible.
A filler for the painting of automobiles has to satisfy a number of requirements. It has to have good levelling properties in order to level out any unevenness in the substrate; it also has to show very good adhesion values both to the substrate and to the finishing paint; and it has to retain those adhesion values, even in the event of overstoving. The filler must be chip-resistant and must produce good results in the condensation/salt spray test in order to support the corrosion-inhibiting effect of the paint. It has to be solvent resistant so that it is not dissolved by the additional layers of paint (e.g., top coats). In addition, it must be resistant to yellowing and support the high film quality of the additional paint layers. EP-A-330,139 describes a process for producing chip resistant coatings in which two layers, namely a chip resistant intermediate primer and a hydrofiller, are applied and are stoved by the wet-on-wet method. This method requires the additional application of a paint coating and is therefore very expensive and time-consuming because it is more favorable to satisfy all of the stated requirements with the same filler layer.
In addition, the carboxyl-functional polyester binders used in EP-A-330,139 have only a limited shelf life because they are vulnerable to hydrolysis.
EP-A-278,394 describes water-dilutable fillers based on polyether urethanes which habe been produced using polyols based on vegetable oils. The polyurethanes produced from these starting materials and hydrophilically modified by anionic dihydroxy and/or diamino compounds are used in particular as additives for aqueous alkyd resin emulsions. To obtain good film properties, therefore, the polyurethanes have to be combined with other binders.
In the preferred process described in EP-A-278,394, polyether polyols and oleochemical polyols are first reacted with a diisocyanate to form an isocyanate-functional prepolymer which is then reacted with the dihydroxy compound containing ionic groups. This process has the disadvantage that at least two and, according to the examples, three different polyol precursors are required for producing the NCO-terminated prepolymer. In addition, only tertiary (i.e., non-NCO-reactive amines, such as triethylamine and N-methyl morpholine) can be used in the described processes. In addition, it is stated in EP-A-278,394 that, when only one polyol as opposed to the polyol composition according to the invention is used, the products obtained have considerably poorer film properties.
An object of the present invention is to provide a stoving filler binder which satisfies all of the previously discussed requirements when the filler is applied in a single layer. An additional object of the present invention is to provide a filler which is easy to produce and has a long shelf life.
Surprisingly, these objects may be achieved by the binder composition and the process for producing a stoving filler which are described in detail hereinafter.